This invention relates to supercritical fluid extraction and more particularly relates to a reciprocating pump for pumping liquid near its supercritical temperature in such systems.
In supercritical fluid extraction, an extraction vessel is held at a temperature above the critical point and is supplied with fluid at a pressure above the critical pressure. Under these conditions, the fluid within the extraction vessel is a supercritical fluid. In one type of apparatus for supercritical extraction, there is a specially constructed extraction vessel within a source of heat and a specially constructed pump for supplying supercritical fluid to the extraction vessel.
One prior art type of pump used for supercritical extraction is the same as a single piston pump used for HPLC. This type of pump has several disadvantages when used for supercritical fluid extraction, which are: (1) a regenerative effect may, under some circumstances, be created in which the heat of compression increases the temperature of the fluid and which in turn increases its compressibility and causes the regenerative effect, which prevents the accurate prediction of flow rate for purposes of control; (2) the usual cams create destructive reverse torques on the pumping cam, gear train and drive motor after the cam passes top dead center because the high compressibility of the liquid in the pump chamber causes the storing of a relatively high amount of energy at high pressures.
Another prior art pump used for supercritical fluid extraction is a multiple cylinder pump of the type now used in HPLC to reduce pulsation. This type of pump, besides being sometimes under some circumstances subject to the problems of single cylinder pumps, is also more expensive and complicated.
In still another prior art pump, a cam for driving the piston that is to pump a supercritical fluid has a slow return stroke intended to reduce destructive forces. This type of pump has a disadvantage insofar as it causes pulsations and delays on time during which fluid is not delivered.
In the prior art pumps, water cooling is usually used or the pumps have very low flow rates. Other prior art discloses cooling of either the inlet fluid or the pumphead. Such prior art discloses cooling just one but not the other. In U.S. Pat. No. 5,087,360, there is disclosed a supercritical fluid extraction system in which both the inlet fluid and pumphead are cooled, but water cooling is used for both.
In supercritical fluid extraction pumps, determination of actual fluid flow rate is a significant problem due to the very high compressibility of fluids used for supercritical applications such as carbon dioxide. The critical temperature of CO.sub.2 is 31.1 degrees C, not much above room temperature. It is difficult to pump fluids near their critical point, a problem not encountered with HPLC pumps. The density of the approximately room temperature liquid (not yet supercritical fluid) leaving the pump is about 11/4 times that of the density of the fluid entering the pump: the compressibility of liquid carbon dioxide is about 11/4 to 1 from 870 psi to 7,500 psi. This compressibility is greater than the liquids used for HPLC. The high compressibility produces an unfortunate regenerative effect. The heat of compression raises the temperature of the fluid, which in turn makes it more compressible. This in turn raises the heat of compression further. The existence of this process makes a priori accurate prediction of flow rate impossible. The prior art cams for driving the plunger of a single-plunger pump for pumping highly compressible liquids such as in a liquid fluid supply for a supercritical extractor have a profile similar to that used in high performance liquid chromatography (HPLC) pumps. However, when using this profile highly compressible fluids at high pressure produce an undesirable and possible destructive reverse torque on the pumping cam, gear train and drive motor after the cam passes top dead center. This is because the high compressibility of the liquid in the pump chamber results in the storage of a relatively large amount of energy at high pressure such as 7500 psi.
One conventional solution to this problem is to use a cam with a slow return stroke. However, the slow return stroke takes up a lot of the cam rotation and it is obvious that liquid can not be delivered from the pump during the return stroke. This causes undesirable mechanical stress in and flow pulsations from the single-plunger pump.
Another conventional solution to this problem is to use a two or more plunger pump as this inherently reduces the pulsations and reduces the reverse torque on the mechanical system since when one head is depressurizing the other pumphead is delivering and is taking up positive torque which subtracts from the reverse torque of the pump it is depressurizing. However, this fix is undesirable because adding a second pumphead decreases reliability because of the increased number of parts and increases the cost of the pump for the same reason.